1. Field of the Invention
This invention relates to the field of photonics, and in particular to a photonic device with a light source, such as a VCSEL (Vertical Cavity Surface Emitting Laser), and a power monitor.
2. Description of Related Art
VCSELs are increasingly used as light sources in fiber optics applications. They have considerable advantages over edge emitting lasers. Some means must generally be provided to control the output of the VCSEL. It is known to reflect a portion of the light back from an output lens onto a detector, typically a PIN diode located adjacent the VCSEL. The VCSEL is usually mounted on top of the PIN diode. The PIN diode generates a control signal dependent on the output power of the VCSEL, and this is used in a feedback loop to control the output power of the VCSEL.
U.S. Pat. No. 5,812,582 describes a technique wherein the VCSEL is mounted on a metallic mask that extends beyond an area covered by the VCSEL. This is necessary because the metallic mask also serves as a bottom contact for the VCSEL, and some means must be provided to supply power into the VCSEL. One problem with this arrangement is that by shadowing part of the diode, the mask decreases the efficiency of the diode since light falling on the mask will not contribute to the output of the diode.
In addition, the mask affects the relationship between the output power of the VCSEL and the magnitude of the photocurrent generated so that the photocurrent is not proportional to the power. This happens because the field is non uniform. The proportion of light output striking the active area of the PIN diode can change with changing power levels. This results in non-linearity in the output of the VCSEL.
According to the present invention there is provided a photonic device comprising a semiconductor substrate having an active photosensitive surface for generating a photocurrent dependent on the intensity of light falling thereon; a metallic contact element formed on said active surface; a light emitting element having a lower electrode, said light emitting element being disposed on said active surface to cover said metallic contact element with said lower electrode bonded to said contact element; a peripheral bonding pad for electrical connection to said contact element; and a doped conductive path within said substrate for establishing electrical connection between said contact element and said peripheral bonding pad.
The doped path is preferably in the form of a doped layer so that current flows uniformly in all directions from a centrally located contact element. In practice, it has been found that upper doped p layer of a PIN diode forming the active surface can have sufficient conductivity to establish electrical connection to the bottom contact of a VCSEL if the current is permitted to flow in all directions from a centrally mounted frame to a peripheral contact pad. This arrangement compensates for the reduced conductivity of a semiconductive material relative to a metal.
The idea of using the doped layer of the PIN diode as a conductive path to the contact pad for the light emitting device results in considerable efficiencies since none of the light falling outside the VCSEL is masked, and also the non-linear problem can be avoided, especially if the VCSEL is symmetrically arranged at the centre of the active surface.
By eliminating the metal contact extending over the active surface, the invention is able to avoid many of the problems of the prior art. The applicants have found surprisingly that by taking advantage of the conductivity of the upper p layer of a PIN diode, this layer can serve the dual purpose of supplying power to the bottom contact of the VCSEL and forming the top surface of the PIN diode. This surface is preferably covered with a passivation layer or antireflective coating in a conventional manner.
In another aspect the invention provides a method of making a photonic device comprising providing a semiconductor substrate having a photosensitive active surface, said semiconductor substrate having a doped layer providing a conductive path between an inner region thereof and a peripheral region; providing a metallic contact element on said inner region of said active surface; providing a bonding pad at the periphery of said active surface; and bonding a light emitting element with a lower contact electrode to said metallic contact so as to cover said contact.